Regarding object tracking and monitoring, indoor location-based service is becoming more and more important nowadays. One popular approach is to use dead-reckoning method to estimate the location of a moving object in real time. Usually, the moving direction and the moving distance are estimated by inertia measurement unit (IMU). However, the performance of moving distance estimation in the dead-reckoning based approach is far from satisfactory, which is the main reason that such indoor navigation systems are still not popular now. Estimating the speed of a moving object in an indoor environment, which can assist the location-based service, is also an open problem and no satisfactory results appear yet. The Doppler effect has been widely applied to different speed estimation systems using sound wave, microwave, or laser light. However, low speed such as human walking speed is very difficult to be estimated using Doppler shift, especially using electromagnetic (EM) waves. This is because the maximum Doppler shift is about Δf=v/cf0, where f0 is the carrier frequency of the transmitted signal, c is the speed of light, and v is the human walking speed. Under normal human walking speed v=5.0 km/h and f0=5.8 GHz, Δf is around 26.85 Hz and it is extremely difficult to estimate this tiny amount with high accuracy. In addition, these methods need line-of-sight (LOS) condition and perform poorly in a complex indoor environment with rich multi path reflections.
Most of the existing speed estimation methods that work well for outdoor environments fail to offer satisfactory performance for indoor environments, since the direct path signal is disturbed by the multipath signal in indoor environments and the time-of-arrival (or Doppler shift) of the direct path signal cannot be estimated accurately. Then, researchers focus on the estimation of the maximum Doppler frequency which can be used to estimate the moving speed. Various methods have been proposed, such as level crossing rate methods, covariance based methods, and wavelet based methods. However, these estimators provide results that are unsatisfactory because the statistics used in these estimators have a large variance and are location-dependent in practical scenarios. For example, the accuracy of one existing speed estimation method can only differentiate whether a mobile station moves with a fast speed (above30 km/h) or with a slow speed (below 5 km/h).
Another kind of indoor speed estimation method based on the traditional pedestrian dead reckoning algorithm is to use accelerometers to detect steps and to estimate the step length. However, pedestrians often have different stride lengths that may vary up to 40% at the same speed, and 50% with various speeds of the same person. Thus, calibration is required to obtain the average stride lengths for different individuals, which is impractical in real applications and thus has not been widely adopted. Object motion detection becomes more and more important nowadays. For example, for security and/or management purposes, a user may want to detect any object motion in the user's house; a manager of a supermarket may want to detect any object motion in the supermarket; and a nurse in a hospital may want to detect any motion of a patient in the hospital.
Existing systems and methods for detecting object motions cannot provide enough accuracy and often lead to false alarms. Existing approaches include those based on passive infrared (PIR), active infrared (AIR) and Ultrasonic. PIR sensors are the most widely used motion sensor in home security systems, which detect human motions by sensing the difference between background heat and the heat emitted by moving people. However, solutions based on PIR sensors are prone to false alarms due to its sensitivity to environmental changes, like hot/cold air flow and sunlight. They are easily defeated by blocking the body heat emission (wearing a heat-insulating full-body suit). Also, their range is limited and need line-of-sight (LOS), and thus multiple devices are needed. In AIR based approaches, an IR emitter sends a beam of IR which will be received by an IR receiver. When the beam is interrupted, a motion is detected. However, this kind of approaches can be easily seen using a regular camera or any IR detection mechanism and also has limited range and thus need LOS. Ultrasonic sensors detect human motion by sending out ultrasonic sound waves into a space and measuring the speed at which they return, and motion can be detected if there exist frequency changes. However, this kind of approaches can be defeated by wearing an anechoic suit. Also, ultrasound cannot penetrate solid objects such as furniture or boxes and cause gaps in detection field. Slow movements by a burglar may not trigger an alarm, too. Thus, existing systems and methods for detecting object motions are not entirely satisfactory.